Facile Fabrication of TiO2 Quantum Dots-Anchored g-C3N4 Nanosheets as 0D/2D Heterojunction Nanocomposite for Accelerating Solar-Driven Photocatalysis

TiO₂ semiconductors exhibit a low catalytic activity level under visible light because of their large band gap and fast recombination of electron–hole pairs. This paper reports the simple fabrication of a 0D/2D heterojunction photocatalyst by anchoring TiO₂ quantum dots (QDs) on graphite-like C₃N₄ (g-C₃N₄) nanosheets (NSs); the photocatalyst is denoted as TiO₂ QDs@g-C₃N₄. The nanocomposite was characterized via analytical instruments, such as powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, t orange (MO) under solar light were compared. The TiO₂ QDs@g-C₃N₄ photocatalyst exhibited 95.57% MO degradation efficiency and ~3.3-fold and 5.7-fold higher activity level than those of TiO₂ QDs and g-C₃N₄ NSs, respectively. Zero-dimensional/two-dimensional heterojunction formation with a staggered electronic structure leads to the efficient separation of photogenerated charge carriers via a Z-scheme pathway, which significantly accelerates photocatalysis under solar light. This study provides a facile synthetic method for the rational design of 0D/2D heterojunction nanocomposites with enhanced solar-driven catalytic activity.


Introduction
Photocatalytic systems that utilize solar-powered oxidation-reduction chemical reactions are attractive for a wide range of applications, including water purification, air purification, and the development of self-cleaning surfaces. In particular, semiconducting nanomaterials have been widely used in photocatalytic systems that harvest abundant solar radiation, thereby allowing the mitigation of environmental pollution [1][2][3]. For the effective application of semiconductors as photocatalysts, a reduction reaction involving photoexcited free electrons must be accompanied by an oxidation reaction involving photoinduced positive holes [4]. Therefore, the valence and conduction bands of a semiconductor photocatalyst should be located at an appropriate energy level (vs. NHE) to provide an ideal bandgap for the broad absorption of solar light [3,5,6].
The photocatalytic activity of TiO 2 semiconductors was first reported by Fujishima and Honda in 1970. Ever since then, TiO 2 materials have been used as the gold standard in fields related to the photocatalytic degradation of pollutants [7][8][9][10]. However, TiO 2 exhibits low visible-light photocatalytic activity levels because of the fast recombination of electron-hole pairs and the large bandgap (3.2 eV), which can only allow absorption in the ultraviolet (UV) region of the electromagnetic spectrum (≤387.5 nm) [11][12][13]. Improving the separation efficiency of photogenerated charge carriers is one of the main challenges limiting the rational design of visible-light-active TiO 2 -based semiconductor photocatalysts.
Conventional TiO 2 semiconductor photocatalysts have a typical diameter of 20-30 nm. In contrast, TiO 2 quantum dots (QDs) have a nanoscale diameter (≤57 nm) and a high surfacearea-to-volume ratio; these properties make the TiO 2 QDs more effective at generating reactive oxygen species, owing to the increased number of active sites for photocatalytic reactions [14,15]. However, a high surface-area-to-volume ratio leads to high surface energy and causes TiO 2 QDs to easily form aggregates, consequently worsening their photocatalytic performance.
Among the different approaches, strategies based on the use of heterojunctions with 0D/2D or 2D/0D/2D dimensions are typically considered to be efficient methods for improving the performance of photocatalysts because they allow the uniform dispersion of 0D nanoscale particles on 2D materials [16][17][18][19][20][21][22][23][24][25][26][27]. Furthermore, the close contact between components of a 0D/2D composite accelerates charge transfer through the heterojunction interface and creates fast-moving channels from the interface to photoactive surface sites [28][29][30][31][32][33]. The ability of excited free electrons to move to a lower energy level inhibits the recombination of electron-hole charge carriers, enabling more efficient charge carrier separation [28,34]. In this regard, nanosized photoactive QDs deposited on the extensive 2D materials not only provide more available active sites, but it also suppress the fast recombination of photoexcited charge carriers.
This paper reports the simple fabrication of TiO 2 QDs-anchored graphite-like C 3 N 4 (g-C 3 N 4 ) nanosheets (NSs), denoted as TiO 2 QDs@g-C 3 N 4 0D/2D heterojunction nanocomposite. In brief, TiO 2 QDs (3-5 nm) were synthesized via the hydrothermal method, and g-C 3 N 4 NSs were prepared via the double calcination of urea in air at 550 • C for 2 h. Then, TiO 2 QDs were combined with thin g-C 3 N 4 NSs via a sonication method, which established intimate contact between the two components of the heterojunction. The final composite was optimized by adjusting the amount of g-C 3 N 4 NSs for a fixed amount of TiO 2 QDs. The optimized composite (TiO 2 QDs@g-C 3 N 4 ) exhibited 5.7-fold and 3.3-fold higher photocatalytic activity levels than those of g-C 3 N 4 and TiO 2 QD alone, respectively. The uniform distribution of the TiO 2 QDs, without severe aggregation, and the favorable electronic structure of the 0D/2D heterojunction effectively reduced the recombination rate of electron-hole pairs, significantly improving the photocatalytic performance of the heterojunction nanocomposite.

Synthesis of TiO 2 QDs and g-C 3 N 4 NSs
A total of 2.5 mL of TiCl 3 (12%) was added to 50 mL of ethanol; the mixture was stirred for 3 h until a transparent solution was obtained. The resultant solution was decanted into an autoclave reactor and maintained at 90 • C for 3 h. Finally, the product was collected and cleaned with anhydrous ethanol via centrifugation for 20 min at a speed of 7000× g rpm to purify the samples, thereby forming TiO 2 QDs. To prepare g-C 3 N 4 NSs, urea (10 g) was finely ground using a mortar and transferred to a crucible, which was covered and wrapped in foil. The crucible was placed in a muffle furnace and was heated twice for 2 h at 550 • C in air [35,36]. The double heating process results in the production of thin g-C 3 N 4 NSs.

Synthesis of the TiO 2 QDs@g-C 3 N 4 Heterojunction
A simple sonication process was conducted to form a heterojunction between TiO 2 QDs and g-C 3 N 4 NSs. Briefly, the as-synthesized TiO 2 QDs (100 mg) and g-C 3 N 4 NSs (10 mg) were dispersed in 40 mL and 20 mL of ethanol under sonication for 30 min, Nanomaterials 2023, 13, 1565 3 of 13 respectively. Two sample solutions were mixed in a beaker and sonicated for 2 h to facilitate the formation of the 0D/2D heterojunction nanocomposite. Zeta potentials of TiO 2 and g-C 3 N 4 samples were measured as 1.05 ± 0.19 mV and −30.57 ± 0.95 mV, respectively, indicating the interplay of electrostatic interactions between them. Then, to purify the samples, the product was collected and cleaned with anhydrous ethanol via centrifugation for 10 min at a speed of 8000× g rpm. The final product of TiO 2 QDs combined with g-C 3 N 4 NSs (so-called TiO 2 QDs@g-C 3 N 4 ) was dried overnight at 60 • C. The whole fabrication procedure is illustrated in Scheme 1.
A simple sonication process was conducted to form a heterojunction betwe QDs and g-C3N4 NSs. Briefly, the as-synthesized TiO2 QDs (100 mg) and g-C3N4 mg) were dispersed in 40 mL and 20 mL of ethanol under sonication for 30 min, tively. Two sample solutions were mixed in a beaker and sonicated for 2 h to facil formation of the 0D/2D heterojunction nanocomposite. Zeta potentials of TiO2 and samples were measured as 1.05 ± 0.19 mV and −30.57 ± 0.95 mV, respectively, ind the interplay of electrostatic interactions between them. Then, to purify the samp product was collected and cleaned with anhydrous ethanol via centrifugation for at a speed of 8000× g rpm. The final product of TiO2 QDs combined with g-C3N4 N called TiO2 QDs@g-C3N4) was dried overnight at 60 °C. The whole fabrication pr is illustrated in Scheme 1.

Photocatalytic Test
The photocatalytic properties of the as-prepared materials (TiO2 QDs, g-C3 TiO2 QDs@g-C3N4) used for the decomposition of methyl orange (MO) were exa All photocatalytic experiments were conducted at 25 °C under simulated solar li each of the materials, 30 mg of the photocatalyst sample was dispersed in 50 m aqueous solution containing 10 ppm of MO dye. To equilibrate adsorption and des of MO on the catalyst, the solution was agitated in the dark for 30 min, followed b sure to light (100 mW/cm 2 ) irradiated by a solar simulator (1000 W) equipped wit lamp with an air mass global 105 filter. At a predetermined time, 1.0 mL of sam extracted from the reaction mixture, transferred to a vacant tube, and centrifug speed of 10,000× g rpm in order to separate the photocatalyst. Two milliliters of D was added to 0.5 mL of the supernatant; a ultraviolet-visible (UV-Vis) spectropho was used to detect the change in absorbance at 463 nm. To confirm the major active in the degradation of MO dye, photocatalytic reactions were conducted in the pre three different scavengers under solar light. Isopropyl alcohol (IPA, 1 mM), benzoq (BQ, 1 mM), and disodium ethylenediamine tetraacetate (EDTA-2Na, 1 mM) are s ing agents for ▪OH, ▪O2 -and h+ species, respectively.

Photocatalytic Test
The photocatalytic properties of the as-prepared materials (TiO 2 QDs, g-C 3 N 4 , and TiO 2 QDs@g-C 3 N 4 ) used for the decomposition of methyl orange (MO) were examined. All photocatalytic experiments were conducted at 25 • C under simulated solar light. For each of the materials, 30 mg of the photocatalyst sample was dispersed in 50 mL of an aqueous solution containing 10 ppm of MO dye. To equilibrate adsorption and desorption of MO on the catalyst, the solution was agitated in the dark for 30 min, followed by exposure to light (100 mW/cm 2 ) irradiated by a solar simulator (1000 W) equipped with Xe arc lamp with an air mass global 105 filter. At a predetermined time, 1.0 mL of sample was extracted from the reaction mixture, transferred to a vacant tube, and centrifuged at a speed of 10,000× g rpm in order to separate the photocatalyst. Two milliliters of DI water was added to 0.5 mL of the supernatant; a ultraviolet-visible (UV-Vis) spectrophotometer was used to detect the change in absorbance at 463 nm. To confirm the major active species in the degradation of MO dye, photocatalytic reactions were conducted in the presence of three different scavengers under solar light. Isopropyl alcohol (IPA, 1 mM), benzoquinone (BQ, 1 mM), and disodium ethylenediamine tetraacetate (EDTA-2Na, 1 mM) are scavenging agents for • OH, • O 2 and h+ species, respectively.

Preparation and Characterization of Samples
Scanning electron microscopy (SEM) images of g-C 3 N 4 and TiO 2 QDs@g-C 3 N 4 samples are shown in Figure 1a,b. The morphology of g-C 3 N 4 shows a sheet-like nanostructure, with an edge thickness of~20 nm. Compared to g-C 3 N 4 NSs, TiO 2 QDs@g-C 3 N 4 showed a rougher surface morphology, retaining the sheet-like structure, but possessing a thicker  Figure 1c,d show the distinct surface morphology of g-C 3 N 4 NSs and TiO 2 QDs@g-C 3 N 4 . The TEM image of g-C 3 N 4 shows a thin sheet-like structure, confirming that it is a two-dimensional (2D) material. In the TEM image of TiO 2 QDs@g-C 3 N 4 , TiO 2 QDs are uniformly distributed over the g-C 3 N 4 NS without severe aggregation; the size of the TiO 2 QDs is in the range of 3-5 nm. In order to confirm the structure of the heterojunction nanocomposite, elemental mapping analysis of TiO 2 QDs@g-C 3 N 4 was performed using an energy-dispersive spectrometer (EDS). As shown in Figure 1e, the mapping images indicate the well-defined spatial distribution of Ti, O, C, and N components in the composite's structure. In addition, the co-existence of Ti and N indicates the formation of the TiO 2 QDs@g-C 3 N 4 heterojunction. The EDS mapping of C components is more distinct than other components are because of the background substrate of carbon tape used for fixing the powder sample, as is usual for SEM-EDS measurements.
showed a rougher surface morphology, retaining the sheet-like structure, but possessing a thicker edge because of the deposition of the TiO2 QDs. The transmission electron microscopy (TEM) images in Figure 1c,d show the distinct surface morphology of g-C3N4 NSs and TiO2 QDs@g-C3N4. The TEM image of g-C3N4 shows a thin sheet-like structure, confirming that it is a two-dimensional (2D) material. In the TEM image of TiO2 QDs@g-C3N4, TiO2 QDs are uniformly distributed over the g-C3N4 NS without severe aggregation; the size of the TiO2 QDs is in the range of 3-5 nm. In order to confirm the structure of the heterojunction nanocomposite, elemental mapping analysis of TiO2 QDs@g-C3N4 was performed using an energy-dispersive spectrometer (EDS). As shown in Figure 1e, the mapping images indicate the well-defined spatial distribution of Ti, O, C, and N components in the composite's structure. In addition, the co-existence of Ti and N indicates the formation of the TiO2 QDs@g-C3N4 heterojunction. The EDS mapping of C components is more distinct than other components are because of the background substrate of carbon tape used for fixing the powder sample, as is usual for SEM-EDS measurements. The crystalline phases and structures of the samples were evaluated via X-ray diffraction (XRD) conducted in the 2θ range of 10°-80°, as shown in Figure 2. XRD peaks of pure g-C3N4 appeared at 2θ = 13° (100) and 27° (002), which are consistent with the XRD peak positions of bulk g-C3N4. XRD patterns of TiO2 QDs and TiO2 QDs@g-C3N4 show the same characteristic diffraction peaks located at 2θ = 25.28°, 37.80°, 48.05°, 53.89°, and 62.68°, which correspond to the (101), (004), (200), (105), and (224) crystal phases of anatase TiO2, respectively [37]. However, the XRD pattern of TiO2 QDs@g-C3N4 showed no peaks of g-C3N4 because of the relatively small amounts of g-C3N4 relative to the contents of TiO2 QDs. The weight ratio of g-C3N4 (10 mg) to TiO2 QDs (100 mg) was approximately 9.1 wt.% in the composite.  [37]. However, the XRD pattern of TiO 2 QDs@g-C 3 N 4 showed no peaks of g-C 3 N 4 because of the relatively small amounts of g-C 3 N 4 relative to the contents of TiO 2 QDs. The weight ratio of g-C 3 N 4 (10 mg) to TiO 2 QDs (100 mg) was approximately 9.1 wt.% in the composite. X-ray photoelectron spectroscopy (XPS) was performed to examine the binding energy and chemical composition of TiO2 QDs@g-C3N4. As shown in Figure 3a, the Ti 2p core-level spectra exhibited two peaks located at 458.7 and 464.5 eV, respectively. These were assigned as 2p3/2 and 2p1/2 peaks, respectively, indicating the sole existence of Ti 4+ because no signals were observed for Ti 2+ and Ti 3+ species [38,39]. In the O 1s core-level spectra shown in Figure 3b, the main peak at 530.0 eV was assigned to the Ti-O bonding of anatase TiO2, and the shoulder peak at 531.1 eV was attributed to the -OH group on the sample's surface [40]. Figure 3c shows that the C 1s spectra de-convoluted into two peaks located at 284.9 and 287.5 eV, which correspond to the C-C and sp 2 -bonded N-C=N groups, respectively [41,42]. Figure 3d displays that the N 1s core-level spectra de-convoluted into three peaks at 398.0, 399.4, and 400.4 eV, which are assigned to the sp 2 -hybridized C=N-C in triazine rings, sp 2 -hybridized C-N(-C)-C or C-N(-C)-H, and C-N-H groups, respectively [43]. The XPS results confirmed the successful construction of a heterojunction nanocomposite of TiO2 QDs and g-C3N4 NSs components.
The XPS spectra of TiO2 QDs and g-C3N4 NSs were compared with those of TiO2 QDs@g-C3N4 to investigate the influence of the interfacial effect between them. As shown in Figure S1a, the C 1s core-level spectra of the samples exhibited two distinct peaks at binding energies of 284.9 and 287.5 eV, corresponding to the C-C and sp 2 -bonded N-C=N groups, respectively [41]. The intensity of C 1s core-level spectra increased due to its binding with g-C3N4 on the surface of TiO2 QDs. As shown in Figure S1b, the N 1s core level spectra for g-C3N4 show strong triple peaks centered at 398.2, 399.4, and 400.3 eV, corresponding to the sp 2 -hybridized C=N-C in triazine rings, sp 2 -hybridized C-N(-C)-C or C-N(-C)-H, and C-N-H groups, respectively [43]. TiO2 QDs@g-C3N4 also exhibited N 1s core-level spectra at the same binding energy range of g-C3N4. These results signify the interfacial interaction between TiO2 QDs and g-C3N4, and g-C3N4 provides a high-quality interface for TiO2 QDs. X-ray photoelectron spectroscopy (XPS) was performed to examine the binding energy and chemical composition of TiO 2 QDs@g-C 3 N 4 . As shown in Figure 3a, the Ti 2p corelevel spectra exhibited two peaks located at 458.7 and 464.5 eV, respectively. These were assigned as 2p 3/2 and 2p 1/2 peaks, respectively, indicating the sole existence of Ti 4+ because no signals were observed for Ti 2+ and Ti 3+ species [38,39]. In the O 1s core-level spectra shown in Figure 3b, the main peak at 530.0 eV was assigned to the Ti-O bonding of anatase TiO 2 , and the shoulder peak at 531.1 eV was attributed to the -OH group on the sample's surface [40]. Figure 3c shows that the C 1s spectra de-convoluted into two peaks located at 284.9 and 287.5 eV, which correspond to the C-C and sp 2 -bonded N-C=N groups, respectively [41,42]. Figure 3d displays that the N 1s core-level spectra de-convoluted into three peaks at 398.0, 399.4, and 400.4 eV, which are assigned to the sp 2 -hybridized C=N-C in triazine rings, sp 2 -hybridized C-N(-C)-C or C-N(-C)-H, and C-N-H groups, respectively [43]. The XPS results confirmed the successful construction of a heterojunction nanocomposite of TiO 2 QDs and g-C 3 N 4 NSs components.
The XPS spectra of TiO 2 QDs and g-C 3 N 4 NSs were compared with those of TiO 2 QDs@g-C 3 N 4 to investigate the influence of the interfacial effect between them. As shown in Figure S1a, the C 1s core-level spectra of the samples exhibited two distinct peaks at binding energies of 284.9 and 287.5 eV, corresponding to the C-C and sp 2 -bonded N-C=N groups, respectively [41]. The intensity of C 1s core-level spectra increased due to its binding with g-C 3 N 4 on the surface of TiO 2 QDs. As shown in Figure S1b, the N 1s core level spectra for g-C 3 N 4 show strong triple peaks centered at 398.2, 399.4, and 400.3 eV, corresponding to the sp 2 -hybridized C=N-C in triazine rings, sp 2 -hybridized C-N(-C)-C or C-N(-C)-H, and C-N-H groups, respectively [43]. TiO 2 QDs@g-C 3 N 4 also exhibited N 1s core-level spectra at the same binding energy range of g-C 3 N 4 . These results signify the interfacial interaction between TiO 2 QDs and g-C 3 N 4 , and g-C 3 N 4 provides a high-quality interface for TiO 2 QDs.
Brunauer-Emmett-Teller (BET) surface area analysis was performed to compare the surface area and pore size distribution of the TiO 2 QDs and TiO 2 QDs@g-C 3 N 4 samples. Figure 4a shows the Type IV isotherm curves of both samples, indicating their mesoporous structures. The BET surface areas of TiO 2 QDs and TiO 2 QDs@g-C 3 N 4 samples were measured as being 367.8, and 352.8 m 2 /g, respectively. The slight decrease in the surface area in the composite could be attributed to the substitution of g-C 3 N 4 (ca. 9.1 wt.%) for high-surface TiO 2 QDs in the total mass. However, the g-C 3 N 4 -based heterojunction provides extensive 2D support for the dispersion of TiO 2 QDs, which is effective to prevent the self-aggregation of TiO 2 QDs [44]. Figure 4b shows the pore size distributions of TiO 2 QDs (average pore diameter of 3.34 nm) and the TiO 2 QDs@g-C 3 N 4 heterojunction (average pore diameter of 5.2 nm), respectively. This larger pore size of the composite can be ascribed to the heterojunction of TiO 2 QDs with g-C 3 N 4 NSs, providing an additional interspace in the range of 20-100 nm. Moreover, TiO 2 QDs@g-C 3 N 4 has a larger pore volume (0.46 cm 3 /g) than that of the TiO 2 QDs (0.32 cm 3 /g). Notably, the increases in the pore size and pore volume may induce the more uniform dispersion of TiO 2 QDs on g-C 3 N 4 NSs, facilitating the separation of photo-excited charged carriers and leading to a high-performance heterojunction photocatalyst. Brunauer-Emmett-Teller (BET) surface area analysis was performed to compare surface area and pore size distribution of the TiO2 QDs and TiO2 QDs@g-C3N4 samp Figure 4a shows the Type IV isotherm curves of both samples, indicating their meso rous structures. The BET surface areas of TiO2 QDs and TiO2 QDs@g-C3N4 samples w measured as being 367.8, and 352.8 m 2 /g, respectively. The slight decrease in the sur area in the composite could be attributed to the substitution of g-C3N4 (ca. 9.1 wt.% high-surface TiO2 QDs in the total mass. However, the g-C3N4-based heterojunction vides extensive 2D support for the dispersion of TiO2 QDs, which is effective to pre the self-aggregation of TiO2 QDs [44]. Figure 4b shows the pore size distributions of T QDs (average pore diameter of 3.34 nm) and the TiO2 QDs@g-C3N4 heterojunction (a age pore diameter of 5.2 nm), respectively. This larger pore size of the composite ca ascribed to the heterojunction of TiO2 QDs with g-C3N4 NSs, providing an additiona terspace in the range of 20-100 nm. Moreover, TiO2 QDs@g-C3N4 has a larger pore vol (0.46 cm 3 /g) than that of the TiO2 QDs (0.32 cm 3 /g). Notably, the increases in the pore and pore volume may induce the more uniform dispersion of TiO2 QDs on g-C3N4 N facilitating the separation of photo-excited charged carriers and leading to a high-per mance heterojunction photocatalyst.

Photocatalytic Activity and Mechanism
The photocatalytic activity of the samples was evaluated based on the degradation of MO dye under simulated solar light. During the photocatalytic reaction, the concentration of residual MO was calculated from the measurement of the time evolution of UV-Vis absorbance at 464 nm during the reaction time. We surveyed the mass fraction of carbon nitride in the heterojunction nanocomposite. According to Figure S2, the level of photo-

Photocatalytic Activity and Mechanism
The photocatalytic activity of the samples was evaluated based on the degradation of MO dye under simulated solar light. During the photocatalytic reaction, the concentration of residual MO was calculated from the measurement of the time evolution of UV-Vis absorbance at 464 nm during the reaction time. We surveyed the mass fraction of carbon nitride in the heterojunction nanocomposite. According to Figure S2, the level of photodegradation (%) of MO dye was maximal at the optimal fraction of g-C 3 N 4 (9.1 wt.%) in the composite, which was prepared by mixing 10 mg of g-C 3 N 4 and 100 mg of TiO 2 QDs under sonication for 2 h. When the mass fraction of g-C 3 N 4 in the composite is lower than the optimal value, the photocatalytic activity of the TiO 2 QDs@g-C 3 N 4 is gradually decreased, while the photocatalytic activity of the TiO 2 QDs@g-C 3 N 4 is significantly decreased when the mass fraction of g-C 3 N 4 is larger than the optimal value, which is probably because of the shielding effect of the excessive g-C 3 N 4 . Thus, TiO 2 QDs@g-C 3 N 4 (9.1 wt.%) was used for further study.
According to the XPS elemental compositions of TiO 2 QDs@g-C 3 N 4 (Table S1), the weight ratio of g-C 3 N 4 to TiO 2 QDs is calculated as 8.47%, which is lower than 9.1% (in that all TiO 2 are loaded with g-C 3 N 4 ). Basically, it is known that g-C 3 N 4 has a molar C/N ratio of 0.75. However, urea-prepared C 3 N 4 has often a lower molar C/N ratio (i.e., the carbon contents are relatively larger than the N contents are) [45]. For this reason, XPS elemental analysis underestimates the proportion of g-C 3 N 4 in the composite. The actual proportion of g-C 3 N 4 may be larger than the calculated value of 9.1% because some TiO 2 QDs are not fully loaded with g-C 3 N 4 NSs. According to the XPS elemental compositions of urea-prepared C 3 N 4 (Table S2), the C/N ratio is calculated as being 1.2 that is larger than 0.75 for the molecular structure of g-C 3 N 4 . Synthesized g-C 3 N 4 is considered to be polymeric C 3 N 4 (p-C 3 N 4 ) with a significant loss of nitrogen atoms. The reason may be attributed to the significant destruction of the layered structure caused by two rounds of calcination at a high temperature (550 • C) for 2 h [46].
As shown in Figure 5a, the as-prepared samples exhibited different degradation efficiencies of MO dye after 120 min. The control solution without a photocatalyst showed no change in the MO concentration, indicating the negligible photolysis of MO under simulated solar light. In contrast, the MO concentration gradually decreased, owing to the catalytic action of g-C 3 N 4 NSs, which exhibited a degradation efficiency of 40.6% after 120 min. TiO 2 QDs and TiO 2 QDs@g-C 3 N 4 exhibited 30% equilibrium adsorption of MO under dark conditions. After solar light irradiation, TiO 2 QDs@g-C 3 N 4 exhibited a degradation efficiency of 95.57%, which was higher than that of TiO 2 QDs (72.76%). The color of MO solution became colorless after 120 min irradiation, as shown in the inset, signifying the almost complete mineralization of MO dye. UV-vis spectra for MO degradation are shown in Figure S3.
A linear plot of ln(C 0 /C) vs. reaction time is shown in Figure 5b according to the pseudo-first-order kinetics of ln(C O /C) = kt. The rate constants of g-C 3 N 4 NSs, TiO 2 QDs, and TiO 2 QDs@g-C 3 N 4 were fitted as k = 4.19 × 10 −3 , 7.31 × 10 −3 , and 2.38 × 10 −2 min −1 , respectively. The rate constant for TiO 2 QDs@g-C 3 N 4 was 5.7-fold and 3.3-fold larger than those of g-C 3 N 4 and TiO 2 QDs, respectively. The electron-hole pair separation efficiency was determined via photoluminescence (PL) analysis at an excitation wavelength of 353 nm. As shown in Figure 5c, TiO 2 QDs@g-C 3 N 4 exhibited the lower PL emission intensity as compared to those of the other samples (g-C 3 N 4 and TiO 2 QDs), indicating an enhanced separation efficiency probably due to suitable band alignment of the 0D/2D heterojunction.
Pristine P25-TiO 2 was tested to assess the efficiency of the newly synthesized materials (TiO 2 QDs and TiO 2 QDs@g-C 3 N 4 ). As shown in Figure S4, the photocatalytic activity of pristine P25-TiO 2 is higher than that of the TiO 2 QDs, but lower than that of TiO 2 QDs@g-C 3 N 4 , i.e., the degradation efficiencies of MO dye over P25 TiO 2 , TiO 2 QDs, and TiO 2 QDs@g-C 3 N 4 are 87%, 73%, and 75% after 120 min irradiation, respectively. This result indicates the superiority of the heterojunction photocatalyst over pristine P25 TiO 2 , which is the gold standard for photocatalytic reactions.  The TiO2 QDs@ g-C3N4 heterojunction with a staggered electronic structure exhibited more photoactivity than TiO2 QDs and g-C3N4 did alone, signifying the interplay of facilitated transport of photo-excited charge carriers. Type-II and Z-scheme heterojunction are the main interfacial transport mechanism for the g-C3N4-based heterojunction [32,33,48]. Before establishing the photocatalytic mechanism, low-energy valence band XPS was performed to identify the valence band (VB) edge potentials of the TiO2 and g-C3N4 components, which were estimated as 3.24 eV and 1.76 eV, respectively ( Figure S6). In addition, scavenger tests indicated that • O2 − and positive hole (h + ) were determined as the main radical species in the decomposition of MO dye.
For the Type II heterojunction, photo-induced holes in the VB position of TiO2 QDS are transferred to the VB position of the g-C3N4, and photo-excited electrons in the CB of The photocatalytic mechanism of the samples was determined by measuring the decomposition rate of MO and by adding three different scavengers (1 mM). Figure 5d shows the degradation efficiencies of MO dye (10 ppm) over TiO 2 QDs@g-C 3 N 4 in the presence of isopropanol (IPA), disodium ethylenediamine tetraacetate (EDTA-2Na), and p-benzoquinone (BQ), which can intercept reactive species of • OH, h + , and • O 2 − , respectively [47]. In relation to normalized MO degradation (100%) without a scavenger, the addition of BQ decreased the degradation efficiency by 83.3%, suggesting that • O 2 − is the dominant active species in the decomposition of MO under solar light. The addition of IPA and EDTA-2Na also led to decreases in the degradation efficiency of 29.7% and 67.5%, respectively, indicating that the positive hole (h + ) is a more active radical species than • OH is. Figure 5d was replotted in the form of C/C o versus time with the kinetic plot from the scavenger test ( Figure S5). It shows the pseudo-first order kinetics that were used to produce the rate constants of k = 1.0 × 10 −3 , 1.89 × 10 −3 , and 7.43 × 10 −3 min −1 for BQ, EDTA-Na, and IPA scavengers, respectively.
In summary, the TiO 2 QDs@g-C 3 N 4 photocatalyst is strongly influenced by the presence of BQ ( • O 2 − scavenger) and EDTA-2Na (h + scavenger). The heterojunction nanocomposite offers a facilitated migration path for excited charge carriers, allowing efficient separation through the heterojunction formation with a staggered electronic structure, which leads to the more activation of • O 2− and h + radicals.
In UV-Vis diffuse reflectance spectroscopy (DRS), absorption spectra of the samples were recorded within the UV-Vis region. According to the UV-Vis DRS results shown in Figure 6a, the absorption edge of TiO 2 QDs@g-C 3 N 4 is shifted to a longer wavelength than that of the TiO 2 QDs (≤~380 nm), generating more charge carriers via heterojunction formation. The products of absorption coefficient (α) and photon energy (hν) are plotted as a function of photon energy to provide information about the electronic and optical properties of the samples (Figure 6b). Based on the Tauc plot, the optical bandgaps of the samples were calculated as 3.02, 3.23, and 3.19 eV for g-C 3 N 4 , TiO 2 QDs, and TiO 2 QDs@g-C 3 N 4 , respectively. The lower bandgap of TiO 2 QDs@g-C 3 N 4 suggests that it has a higher capacity for light harvesting, bestowing the beneficial effect of the 0D/2D heterojunction toward expanding the wavelength of light absorption.  The TiO2 QDs@ g-C3N4 heterojunction with a staggered electronic structure exhibited more photoactivity than TiO2 QDs and g-C3N4 did alone, signifying the interplay of facilitated transport of photo-excited charge carriers. Type-II and Z-scheme heterojunction are the main interfacial transport mechanism for the g-C3N4-based heterojunction [32,33,48] Before establishing the photocatalytic mechanism, low-energy valence band XPS was performed to identify the valence band (VB) edge potentials of the TiO2 and g-C3N4 components, which were estimated as 3.24 eV and 1.76 eV, respectively ( Figure S6). In addition scavenger tests indicated that • O2 − and positive hole (h + ) were determined as the main radical species in the decomposition of MO dye.
For the Type II heterojunction, photo-induced holes in the VB position of TiO2 QDS are transferred to the VB position of the g-C3N4, and photo-excited electrons in the CB of The TiO 2 QDs@ g-C 3 N 4 heterojunction with a staggered electronic structure exhibited more photoactivity than TiO 2 QDs and g-C 3 N 4 did alone, signifying the interplay of facilitated transport of photo-excited charge carriers. Type-II and Z-scheme heterojunction are the main interfacial transport mechanism for the g-C 3 N 4 -based heterojunction [32,33,48]. Before establishing the photocatalytic mechanism, low-energy valence band XPS was performed to identify the valence band (VB) edge potentials of the TiO 2 and g-C 3 N 4 components, which were estimated as 3.24 eV and 1.76 eV, respectively ( Figure S6). In addition, scavenger tests indicated that • O 2 − and positive hole (h + ) were determined as the main radical species in the decomposition of MO dye.
For the Type II heterojunction, photo-induced holes in the VB position of TiO 2 QDS are transferred to the VB position of the g-C 3 N 4 , and photo-excited electrons in the CB of g-C 3 N 4 are transferred to the CB of TiO 2 QDs. In the case of Type II, the valence band edge potential is not sufficient to form hydroxyl radicals from water via a reaction with positive holes because the VB position of g-C 3 N 4 is higher than the potential of the H 2 O/•OH couple (2.8 V vs. NHE) [49]. In the Z-scheme, however, the photo-induced holes tend to stay in the more positive VB of TiO 2 QDs, which is sufficient to produce hydroxy radicals from water, and photo-excited electrons are accumulated in the more negative CB of the g-C 3 N 4 , which maintains the high redox powers of free charge carriers. In this regard, the Z-scheme mechanism is more appropriate to interpret the photocatalytic activity of TiO 2 QDs@g-C 3 N 4 with highly enhanced photocatalytic activity.
In summary, the TiO 2 QDs@ g-C 3 N 4 heterojunction system exhibited more enhanced photoactivity than TiO 2 QDs and g-C 3 N 4 did alone. Photoexcited electrons in the conduction band (CB) of TiO 2 (0.01 eV vs. NHE) are transferred to the VB of g-C 3 N 4 (1.76 eV vs. NHE) via a Z-scheme pathway and further excited to the CB of g-C 3 N 4 (−1.28 eV vs. NHE) with high reducing power, whereas positive holes remained in the VB of TiO 2 QDs (3.24 eV vs. NHE), which can directly produce hydroxy radicals from water. The 0D/2D Z-scheme heterojunction causes the efficient separation of photogenerated charge carriers with high redox power, significantly enhancing solar-driven photocatalysis [8,32,33]. The photocatalytic mechanism underlying the solar-driven photocatalysis of TiO 2 QDs@g-C 3 N 4 is illustrated in Figure 7. NHE) via a Z-scheme pathway and further excited to the CB of g-C3N4 (−1.28 eV vs. NHE) with high reducing power, whereas positive holes remained in the VB of TiO2 QDs (3.24 eV vs. NHE), which can directly produce hydroxy radicals from water. The 0D/2D Zscheme heterojunction causes the efficient separation of photogenerated charge carriers with high redox power, significantly enhancing solar-driven photocatalysis [8,32,33]. The photocatalytic mechanism underlying the solar-driven photocatalysis of TiO2 QDs@g-C3N4 is illustrated in Figure 7. The photocatalytic stability of TiO2 QDs@g-C3N4 was tested by measuring the degradation efficiency of MO dye after 120 min using recycled photocatalysts. The tested sample was collected via centrifugation after the reaction. After washing it with water and ethanol several times, the recovered sample was dried in an oven for the subsequent photocatalytic reaction. The photocatalytic results for a total of four cycles are shown in Figure  S7. The photocatalytic activity decreased by 2.3% even after four recycling test. TiO2 QDs@g-C3N4 exhibited photocatalytic stability under repeated solar light exposure, indicating the high structural stability of the heterojunction nanocomposite.

Conclusions
In this paper, we reported the facile fabrication of a TiO₂ QD-anchored g-C₃N₄ NSs, TiO₂ QDs@g-C₃N₄ 0D/2D heterojunction photocatalyst. In the TEM image of TiO2 QDs@g-C3N4, TiO2 QDs (3-5 nm) were uniformly distributed over g-C3N4 NSs, without severe The photocatalytic stability of TiO 2 QDs@g-C 3 N 4 was tested by measuring the degradation efficiency of MO dye after 120 min using recycled photocatalysts. The tested sample was collected via centrifugation after the reaction. After washing it with water and ethanol several times, the recovered sample was dried in an oven for the subsequent photocatalytic reaction. The photocatalytic results for a total of four cycles are shown in Figure S7. The photocatalytic activity decreased by 2.3% even after four recycling test. TiO 2 QDs@g-C 3 N 4 exhibited photocatalytic stability under repeated solar light exposure, indicating the high structural stability of the heterojunction nanocomposite.

Conclusions
In this paper, we reported the facile fabrication of a TiO 2 QD-anchored g-C 3 N 4 NSs, TiO 2 QDs@g-C 3 N 4 0D/2D heterojunction photocatalyst. In the TEM image of TiO 2 QDs@g-C 3 N 4 , TiO 2 QDs (3-5 nm) were uniformly distributed over g-C 3 N 4 NSs, without severe aggregation. The XRD results for TiO 2 QDs@g-C 3 N 4 showed the same characteristic diffraction peaks at 2θ = 25.28 • , 37.80 • , 48.05 • , 53.89 • , and 62.68 • , corresponding to the (101), (004), (200), (105), and (224) crystal phases of anatase TiO 2 , respectively. Furthermore, the EDS and XPS data confirmed the successful construction of the 0D/2D heterojunction nanocomposite and the coexistence of TiO 2 and g-C 3 N 4 components. The performance of the as-prepared samples (TiO 2 QDs, g-C 3 N 4 NSs, and TiO 2 QDs@g-C 3 N 4 ) toward MO decomposition under simulated solar light was analyzed. The TiO 2 QDs@g-C 3 N 4 photocatalyst showed MO degradation, with an efficiency of 95.57%, which was 3.3-fold and 5.7-fold higher than those of TiO 2 QDs and g-C 3 N 4 , respectively. The 0D/2D TiO 2 QDs@g-C 3 N 4 photocatalyst possessed a staggered electronic structure that facilitated the efficient separation of charge carriers via a Z-scheme pathway, significantly enhancing solar-driven photocatalysis. The work proposed a simple method for fabricating highperformance 0D/2D heterojunction photocatalysts for environmental purification and energy conversion applications.